This invention relates to steam generation steam cycles, and more specifically to a method for determining the existence of phosphate hideout in a steam generating steam cycle so that the nature of the impact of the phosphate hideout on pH and PO.sub.4 concentration in the boilerwater of the steam generating steam cycle can be ascertained.
It has long been a well-known fact in the industry that corrosion in utility steam generators is an area of significant concern to both the manufacturers and users of such equipment. To this end, billions of dollars are reportedly being spent annually in the power generation industry alone in an effort to alleviate problems which are alleged to be caused by corrosion. Significant strides have heretodate been made in attempting to minimize and, in some cases, to eliminate the effects of corrosion. Yet, despite the millions of dollars which to date have been spent for research and development on new materials and on developing operating practices to deal with corrosion and its effects, there are areas in which there still exist a need for improvement.
Reportedly, as high as 50% of the forced outages that are experienced by fossil-fuel steam generators are estimated to be corrosion-related. Such forced outages of steam generators when translated into dollars and cents have costs associated therewith which are deemed to be of the magnitude of $500 million dollars annually. The two major causes of steam generator forced outages, i.e., the two most vulnerable portions of the steam generator steam cycle, have been found to be the furnace waterwalls and the steam circuits, which represent approximately 40% and 30%, respectively, of the failures that result in the forced outage of a fossil-fired steam generator. Additionally, of the major turbine problems that are experienced at utility steam generating installations, it has been found that two-thirds of them are associated with long-term steam purity upsets. A primary cause of corrosion-induced problems in these units is related to the water and steam-side chemistry environments. Prime candidates for failure when chemistry upsets occur are both thin-walled and thick-walled components. By way of exemplification and not limitation hydrogen and caustic damage are directly related to improper boilerwater pH control, while oxygen pitting and overheating, stemming from the deposition of corrosion products, result from the inability to control oxygen and/or pH. Per unit, forced outages resulting from these and other corrosion-related failures can be quite costly, ranging from $120,000 to $720,000 per day for a 500 MW unit. Lost generating time and subsequent purchase of power for resale frequently constitute the major portions of outage costs. Consequently, minimizing or eliminating these types of occurrences can have both short-term and long-term implications for reducing overall operating and maintenance expenses.
Economics have also played a role in the increasing emphasis which is being placed on corrosion mitigation. Namely, as a result of U.S. economic conditions over the past 5-7 years, the task of forecasting load growth and electricity demand have become fret with uncertainties. This has had the effect of placing utilities in the position of having to make difficult decisions insofar as concerns arriving at a choice between the purchase of new equipment and the refurbishment of used equipment. To this end, programs have been initiated, including but not limited to life extension studies, which have for their objective the identification of the existence of deficiencies both in terms of equipment and in terms of operating practices which if modified and/or updated would have the effect of restoring unit integrity and/or of enabling operations to be maintained for extended periods of time at an acceptable level. Also, government and industry organizations in addition have instituted programs which are designed to be operative to aid in effectuating the assessment of steam generator integrity. Many of these programs are related directly to the prevention of corrosion. Furthermore, it is known that much of the funding which is being expended in order to accomplish the implementation of the recommendations that have been generated in the course of performing such programs is being spent on the replacement and/or refurbishing of corrosion-damaged components.
New and better ways are being sought to avoid past problems and to assure increased steam generator availability and reliability. To the extent that a steam generator's operating mode changes from base-loaded to cycling-operation, this task of increasing steam generator availability and reliability becomes even more difficult. As such, unless increased emphasis is placed on the steam generator's cycle chemistry environment, it can almost be guaranteed that corrosion-induced problems will occur.
The responsibility for implementing appropriate water technology practices, which can best meet the operational chemistry requirements of a given steam generating installation, rest with the operator of the steam generator. In turn the steam generator operators strive to meet these requirements by establishing monitoring, interpretation, control and trending methods which will work within the particular environment that is found to be present at a given steam generating installation. The methods which are used in this regard by the steam generator operator generally are adapted from generic guidelines which have been established by the various suppliers of the equipment which is being utilized.
By way of exemplification and not limitation, it will be assumed for purposes of the discussion which follows that the type of application which is the focus of attention is that of a high pressure steam cycle of the sort that one associates with a utility-type steam generator. In such an application, since the major sections of the cycle are coupled together the water chemistry parameters for each of the sections must be compatible. As an example, consider that the steam turbine manufacturer has set limits for constituents contained in the steam. These limits in turn function as constraints on boilerwater chemistry and also on feedwater chemistry when used as desuperheating spraywater. In addition, limits established for boilerwater chemistry function as another constraint on feedwater chemistry. It should thus be readily apparent that when contamination occurs such as from condenser leakage the entire cycle is affected. Finally, startups and load changes are also known to cause perturbations in the operational chemistry requirements of the cycle.
Continuing, there are to be found in the prior art the result of studies that have been conducted heretofore which contain findings derived from an examination of the nature of the monitoring points that have been employed for purposes of effectuating water chemistry monitoring of a high pressure steam cycle of the sort that is associated with a utility-type steam generator as well as from an examination of the frequency with which samples are normally taken at each monitoring point. Such studies encompass samples which have for water chemistry monitoring purposes been taken from the condensate/feedwater system, from the boilerwater and from the steam. With respect to the examination of these samples, the parameters that have been analyzed include pH, specific and cation conductivity, oxygen, hydrazine, silica, sodium, phosphate, chloride, iron and copper. The findings of these studies further reveal that sampling frequency varies on the one hand from continuous monitoring to on the other hand grab samples taken on the order of four times a year.
A detailed list of guidelines for monitoring and controlling steam cycle water chemistry has been compiled by one of the industry organizations. Such guidelines will undoubtedly serve as an excellent reference for steam generator operators. That is, the steam generator operators will be able to utilize these guidelines for purposes of developing a plan that has been customized to meet the requirements of their particular steam generating facility. It is known that at present not many steam generating installations utilize the full complement of possible monitoring points that are available. In addition, it is known that at not many steam generating installations are samples taken with the frequency that it is believed they should be. To this end, the present practice is to select for monitoring one or more key parameters which are perceived to be sensitive indicators of the steam cycle contamination, and to effect the monitoring thereof through the use of strip chart recorders and alarms which are found located in the control room at the steam generating installation. Other information is collected on log sheets which are reviewed periodically in order to detect trends and/or to assist in the identification of problem areas. The information which is compiled from such sources can in turn then be utilized for purposes of determining what, if any, control actions need to be taken. The actual implementation of such control actions will be effected, depending on a consideration of factors such as system preferences and shift coverage, either by the operators or by the chemistry laboratory technicians. Normally, such control actions are based on written instructions and/or consultation with the chemist who is assigned to the steam generating facility in question. Unfortunately, however, the task of establishing proper control over steam cycle chemistry is becoming more difficult both as the impact of trace contamination on the equipment being employed in the steam cycle becomes clearer, and as improvements in analytical measurements permit the detection of sub-part per billion concentrations of contaminants.
For purposes of accomplishing the monitoring function as well as for purposes of presenting the information derived from such monitoring, the trend in the case of steam cycle chemistry as in the case of many other things these days is toward computerization. Computerization as referred to herein is meant to refer to the use of mainframe as well as the use of desk top computers. By using computers, it is possible to gain rapid access to large amounts of chemistry data while at the same time permitting this data to be presented in an easy-to-understand format.
From the foregoing discussion it can, therefore, be clearly seen that the chemistry personnel at a steam generating installation face a difficult task in having to first assimilate a large body of data and then in having to draw conclusions on a real-time basis from this large body of data. Further, it is a requirement of these chemistry personnel that they possess an understanding of long-term trends and system performance so that they are in a position to meaningfully interpret this large body of data. For purposes of controlling both the short-term and the long-term mechanisms which can cause corrosion damage in a steam generating steam cycle it is necessary that the factors enumerated above be considered. To this end, a need has been evidenced in the prior art for a new and improved method for determining the existence of phosphate hideout in a steam generating steam cycle such that the nature of the impact of phosphate hideout on pH and the PO.sub.4 concentration in the boilerwater of the steam generating steam cycle can be ascertained.
It is, therefore, an object of the present invention to provide a new and improved method for determining the existence of phosphate hideout in a steam generating steam cycle.
It is another object of the present invention to provide such a method whereby in accord therewith from the concentration of Na and PO.sub.4 that is provided to the steam generating steam cycle there is determined what the input of Na and PO.sub.4 to the steam generating steam cycle is.
It is still another object of the present invention to provide such a method whereby in accord therewith there is determined what the output of Na and PO.sub.4 resulting from blowdown is from the steam generating steam cycle.
A further object of the present invention is to provide such a method whereby in accord therewith there is determined what amount of Na and PO.sub.4 is consumed in the steam generating steam cycle due to contamination.
A still further object of the present invention is to provide such a method whereby in accord therewith there is determined the change in the amount of PO.sub.4 that is solubilized in the boilerwater of the steam generating steam cycle.
A yet still further object of the present invention is to provide such a method whereby in accord therewith by adding together the output of PO.sub.4 due to blowdown from the steam generating steam cycle and the amount of PO.sub.4 that is consumed in the steam generating steam cycle due to contamination and the change in the amount of PO.sub.4 that is solubilized in the boilerwater of the steam generating steam cycle there is determined what the loss of PO.sub.4 from the steam generating steam cycle is.
Yet another object of the present invention is to provide such a method whereby in accord therewith as a result of subtracting from the input of PO.sub.4 to the steam generating steam cycle the loss of PO.sub.4 from the steam generating steam cycle there is determined whether there exists phosphate hideout and, if so, there is also determined what the magnitude of the phosphate hideout is as well as what impact the phosphate hideout will have on the pH of and the PO.sub.4 concentration in the boilerwater of the steam generating steam cycle.
Yet still another object of the present invention is to provide such a method for determining the existence of phosphate hideout in a steam generating steam cycle that is advantageously characterized by the relative ease with which the subject method can be practiced as well as by the relative inexpensiveness to do so.